( 762 ) 
to make it possible the closure of the piezometer tube by a U-tube 
with mercury was devised (see Comm. n° 50). In our case however 
it is absolutely necessary because the screw joints gy. 93 (Comm. 
n° 69) can develope a leak. 
1. The normal volume is in principle determined in the manner 
employed by ScHALKWIJK Comm. n° 70 V. 
The bath used by him was not large enough to enclose the whole 
piezometer, but only tbe whole of the large volume. The graduated 
stem hb, and the piezometer bulb were thus outside the bath and the 
constancy of the temperature of these parts had to be otherwise 
provided for. Had we sought the same accuracy as required by 
SCHALKWIJK, a new waterbath of the total length of the piezometer 
would have been required. 
However as we could not from the other measurements expect such 
a high accuracy, we were satisfied here also with a somewhat less 
degree and found that the temperatures of the divided stem b, and of 
the piezometer bulb f} could be sufficiently determined by the use 
of simple water baths. Owing to the great length of the large reser- 
voir and to its near equality with that of the bath for constant 
temperature, it was particularly necessary to avoid real differences 
in temperature between the bottom and top of the bath. To attain 
this it was necessary to arrange that the temperature of the room 
should not differ more than a few degrees from that of the bath. 
The latter was thus set at a temperature that could be reasonably 
attained in the room, and the room kept as near as pussible to 
this temperature. 
A small change was also made in the xylene regulator. The 
tube ¢ (see Plates, Comm. n°. 70, III) which was formerly of glass 
was now brass above and steel where it entered the mercury, the 
requisite form could thus be given to the end and narrower tubes 
employed. In this way a greater sensitiveness was attained com- 
bined with an adjustability of some five degrees by merely moving 
the upper brass tube up or down in its brass support, without the 
troublesome removal or addition of mercury which was necessary 
before and which would also be required for greater changes. 
The thermostat was also raised some 1.5 M. to obtain a quicker 
flow, which was shown to be desirable before (Comm. ns 10, NIE 
In the earlier measurements the pressure was read by a controlled _ 
aneroid at intervals. When however the barometric height is changing’ 
rapidly a larger error may arise from this than is allowable. In 
the later measurements the arrangement of Comm. n°. 60 was 
employed, the joint b (Plate II, Comm. n°. 69) being connected by 
